Simple, easy-to-clamp jig aligns the proper drills and taps so you get perfectly-centered, cleanly-threaded scope base holes without all the extra setup time and hassle. Based on the principle of sliding parallels, this jig places locking, hardened drill bushings in one of three different hole spacing patterns. Gives the versatility to drill and tap for almost any scope mount base. Mfg: Williams Gun Sight SPECS: Steel & aluminum, black finish. Approximately 5 1/2″ (14cm) long, 2 3/4″ (7cm) wide. Fits receivers between .630″ and 1.440″ in diameter. Includes #31 (6-48) and #28 (8-40) drill and tap bushings.. Drill Perfectly Centered Holes On Round, Flat Or Octagon Receivers